/Engine/Source/Runtime/Engine/Classes/Kismet/KismetSystemLibrary.h
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The number of active actions when BeginTransaction was called (values greater than 0 indicate that there was already an existing undo transaction in progress), or -1 on failure.
static int32 BeginTransaction ( const [FString](API\Runtime\Core\Containers\FString) & Context, [FText](API\Runtime\Core\Internationalization\FText) Description, [UObject](API\Runtime\CoreUObject\UObject\UObject) * PrimaryObject )